Transmit beamforming is a technique for improving wireless communications between a transmitting (TX) device and a receiving (RX) device by leveraging known properties of the wireless channel (e.g., channel state information (CSI)). In conventional beamforming, the TX device utilizes multiple antennas to focus energy (e.g., data signals) toward the RX device. For example, each of the antennas of the TX device may transmit the same wireless signal with different phase offsets to “steer” the signal in the direction of the RX device (e.g., as determined from the CSI). Beamformed signals converge (e.g., combine) along the steered path and interfere with (e.g., cancel) one another outside of the steered path. Thus, the effectiveness of beamforming may depend on the accuracy of the CSI.
The IEEE 802.11ac standards describe an explicit “sounding” technique that may be performed by a TX device to acquire CSI. The TX device transmits a “sounding packet” (e.g., a null data packet (NDP)) to an RX device to elicit a channel measurement. Upon receiving the sounding packet, the RX device generates a channel estimate for the wireless channel based on training data included with the sounding packet. The RX device then feeds back the channel estimate (e.g., CSI) to the TX device. Because beamforming techniques typically require multiple antennas to implement, the IEEE 802.11ac standards describe the sounding protocol in the context of multiple spatial streams. More specifically, the sounding packet defined by the IEEE 802.11ac standards is formatted for transmission via two or more spatial streams.
Single-antenna TX devices may be incapable of transmitting via multiple spatial channels (e.g., due to lack of antenna diversity). As a result, conventional single-antenna TX devices may be unable to take advantage of existing beamforming techniques.